The isomer of Vitamin A acid, 13-cis Vitamin A acid has become a valuable pharmaceutical due to its use in treating acne. However, this isomer has been very difficult to produce. Pattenden and Weedon, J. Chem. Soc. (C), 1984-1997 [ ] 968] have disclosed a procedure for preparing 13-cis Vitamin A acid by reacting a C.sub.15 -Wittig salt, i.e. [3-methyl-5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-2,4-pentadienyl]-triphenyl phosphonium salt and a C.sub.5 -butenolide, i.e. 5-hydroxy-4-methyl-2-5[H]-furanone to produce a mixture of the cis isomers of retinoic acids. This process suffered from the drawback that the yield of the mixture of cis isomers of retinoic acid was poor. Furthermore, it has been difficult to convert by isomerization this mixture of isomers into the specific isomer 13-cis retinoic acid. This is especially true since the isomers produced by this process contain a cis configuration about both the 11- and 13-positions. It has been very difficult to selectively isomerize the 11-cis double bond without isomerizing the 13-cis double bond. Many of the isomerization techniques used have been relatively ineffective in selectively isomerizing the 11-cis double bond without affecting the 13-cis double bond. Any isomerization of the 13-cis double bond reduces the yield of the 13-cis Vitamin A acid.
Therefore, it has been desired to provide a process where the various cis isomers are produced by the Wittig reaction in high yields and the cis isomers are selectively isomerized without affecting the 13-cis double bond to obtain the 13-cis isomer of Vitamin A acid in high yields and in pure form.